Why is rendering with cpu added only mildly faster even though it uses so much more power?
You haven't given your hardware, but it's possible your CPU just doesn't have that much to add by comparison.
There is still some CPU load when doing a GPU render, so everything leftover is what you have to dedicate to additional CPU rendering.  Your GPU has dedicated silicon to do significant portions of the math involved with rendering, and that's what typically makes it render tiles much faster, then it hands the rest off to the CPU.
A couple of things:  If you're using large tiles, CPUs have trouble handling those.  Enable the Auto Tile Size addon, and it should find a good compromise for hybrid rendering.
You can also try rendering through command line, I've heard this reduces load on the system because you're not constantly integrating and redrawing the image for the render tab.